The semiconductor industry makes use of relatively large tube furnaces, with the tubes being several feet long and from several inches to a foot or more in internal diameter. These furnaces are used to heat semiconductor wafers which are generally held in quartz tubes within the furnaces. The furnaces must generally be able to heat up to relatively high temperatures, usually from about 300.degree. C. to about 1300.degree. C. A considerable amount of electrical energy is utilized to heat these furnaces to operating temperature and to keep them at that temperature.
The tube furnaces usually utilized in the semiconductor industry have heating elements which consist of a spirally wound coil of very heavy wire, perhaps 1/4 inch to 3/8 inch in diameter, and having the axis of the coil coincident with the axis of the tube furnaces with the spirally coiled wire being embedded within insulating material. Generally, the interiors of the furnaces have been the natural color of the ceramic material which is white or off-white.
Such furnaces as are described above have a number of drawbacks. In order to use such heavy wire, it has been necessary to operate at relatively high amperages, generally above 80 amps. This has required the use of relatively expensive stepdown transformers and silicon controlled rectifiers. Furthermore, since the spiral heating elements are generally embedded within an insulating material, a good deal of the energy produced by the elements is, in essence, wasted through dissipation into the insulating material and into the surrounding space. Also, primarily due to the weight of the wire, the furnaces heat up and cool off very slowly and tend to stabilize at a desired temperature relatively slowly. Still further, the relatively light colored interior of the furnace tubes is not of maximum efficiency for transferring energy to objects held within the furnace tube.
Somewhat improved heating elements have been recently developed which utilize considerably thinner heating wires in a sinuated configuration, which wires are near the surfaces of their embedding insulation on the furnace side of such elements. Furthermore, the heating surfaces of such heating elements have generally been colored black so as to improve thermal energy transfer. The size of the heating wire which has been successfully usable in such new heating elements has generally been no greater than about 18 gauge (about 1 mm diameter wire) and such elements have not been sufficient for providing the very high heat needed for such applications as tube furnaces for relatively long periods of time of use. Indeed, such heating elements have generally been available only as flat units for other uses rather than as tubularly shaped units for tube heating furnace use.
It would be very advantageous if a new and lightweight heating element could be provided which would be readily adaptable for use in tube furnaces, would greatly improve the heating efficiency of such tube furnaces with concurrent energy savings, would heat up and cool down quickly, would stabilize relatively quickly at a desired temperature, would provide the high temperatures necessary within tube furnaces and other furnaces, and would last for long periods of time when delivering such heat flux.